1. Field of the Invention
The present invention relates to an apparatus for detecting decrease in tire air-pressure and a method thereof. More particularly, it relates to an apparatus for detecting decrease in tire air-pressure and a method thereof which is capable of reliably determining decrease in internal pressure of a tire, especially of a vehicle which is equipped with a limited slip differential device (LSD).
2. Description of Related Art
It has been conventionally known that decrease in tire air-pressure results in a smaller dynamic load radius of a tire and in a faster revolution speed than a tire of normal air-pressure. For instance, in Japanese Unexamined Patent Publication No. 149119/1995 a method has been proposed for detecting decrease in internal pressure based on relative differences in the number of revolutions of tires. Further, since the number of revolutions of tires are influenced by turning movements, acceleration/deceleration, load or speed of a vehicle, various measures have been taken to eliminate influences thereof.
However, there are vehicles today which are equipped with a limited slip differential device located at a differential gear of a driving shaft for the purpose of improving the driving performance at the time of cornering or the like. According to the mechanism of the limited slip differential device, differentiation is restricted until a differential torque exceeds a set value and the right and left driving wheels are rotated at equal speeds. Due to this arrangement, influences of decreased air-pressure are not reflected in the number of revolutions whereby decrease in pressure cannot be detected based on relative comparison of the number of revolutions.
The present invention has been made in view of these facts, and it is an object thereof to provide an apparatus for detecting decrease in tire air-pressure and a method thereof which can reliably determine a decrease in internal pressure of a tire even in a vehicle which is equipped with a limited slip differential device.